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Anisotropic Nanoantenna-Based Magnetoplasmonic Crystals for Highly Enhanced and Tunable Magneto-Optical Activity Nicolò Maccaferri, Luca Bergamini, Matteo Pancaldi, Mikolaj Schmidt, Mikko Kataja, Sebastiaan van Dijken, Nerea Zabala, Javier Aizpurua, and Paolo Vavassori Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b00084 • Publication Date (Web): 11 Mar 2016 Downloaded from http://pubs.acs.org on March 13, 2016
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Anisotropic Nanoantenna-Based Magnetoplasmonic Crystals for Highly Enhanced and Tunable MagnetoOptical Activity Nicolò Maccaferri1‡*, Luca Bergamini2,3‡, Matteo Pancaldi1‡, Mikolaj K. Schmidt3, Mikko Kataja4, Sebastiaan van Dijken4, Nerea Zabala2,3, Javier Aizpurua3, and Paolo Vavassori1,5* 1
2
CIC nanoGUNE, E-20018 Donostia-San Sebastian, Spain
Dept. of Electricity and Electronics, Faculty of Science and Technology, UPV/EHU, E-48080 Bilbao, Spain
3
Materials Physics Center CSIC-UPV/EHU and Donostia International Physics Center, DIPC, E20018 Donostia-San Sebastian, Spain 4
NanoSpin, Department of Applied Physics, Aalto University School of Science, FI-00076 Aalto, Finland 5
Ikerbasque, Basque Foundation for Science, E-48013 Bilbao, Spain
KEYWORDS: magneto-optical Kerr effect, lattice surface modes, plasmonic anisotropy, magnetoplasmonics
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ABSTRACT. We present a novel concept of a magnetically tunable plasmonic crystal based on the excitation of Fano lattice surface modes in periodic arrays of magnetic and optically anisotropic nanoantennas. We show how coherent diffractive far-field coupling between elliptical nickel nanoantennas is governed by the two in-plane, orthogonal and spectrally detuned plasmonic responses of the individual building block, one directly induced by the incident radiation and the other induced by the application of an external magnetic field. The consequent excitation of magnetic field-induced Fano lattice surface modes leads to highly tunable and amplified magneto-optical effects as compared to a continuous film or metasurfaces made of disordered non-interacting magnetoplasmonic anisotropic nanoantennas. The concepts presented here can be exploited to design novel magnetoplasmonic sensors based on coupled localized plasmonic resonances, and nanoscale metamaterials for precise control and magnetically-driven tunability of light polarization states.
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Metallic nanostructures are known to exhibit collective electronic oscillations, so-called localized surface plasmon resonances (LSPRs),which determine their optical response in the visible and near-infrared spectral range and lead to extreme confined and enhanced electromagnetic fields at the nanoscale.1-4 Since the last decade conventional metallic materials, such as gold and silver, have been used for creating electromagnetically coupled nanostructured systems, dubbed metamaterials, with novel optical properties emerging from the subwavelength confinement of light.5 The optical response of these systems can be tailored by designing the response of the individual units as well as the electromagnetic coupling between them. Besides its fundamental importance, manipulation of light at the nanoscale is of great interest in applied research fields for the prospect of applications in real-life6, such as energy harvesting and photovoltaics7,8, wave-guiding and lasing9,10, optoelectronics11 and biochemistry12,13. Among the exciting phenomena arising from light manipulation at the nanoscale, an emerging perspective relies on the possibility to create multifunctional metamaterials, where the optical properties can be actively tuned by exploiting the synergistic interplay between different physical properties of the constituents. The plasmonic response can be modified for instance through the excitation and tuning of lattice phonons (acousto-plasmonics) or spins of the electron gas in the material (magneto-plasmonics).14-16 Amongst them, magneto-optical (MO) metamaterials based on magnetic plasmonic nanoantennas offer promising routes to actively manipulate light properties at the nanoscale using magnetic fields. The interplay between electronic charge oscillations and their magnetic response controlled by an external stimulus such as a static or dynamic magnetic field, has been already shown to bring novel and unexpected optical effects.17,18 In more detail, pure ferromagnetic nanoantennas support LSPRs and display a magnetic field-induced optical activity, namely a MO activity (MOA), which can be exploited to achieve an active and remote
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magnetic field control of light polarization over the entire visible and near-infrared spectral regions.19-21 It has been recently shown that the field-induced MOA of pure ferromagnetic nanoantennas can produce localized plasma oscillations along directions perpendicular to the LSPR that is directly excited by the incident light, owing to the inherent spin-orbit coupling (SOC) in the constituent material. For instance, a radiation impinging normally on a ferromagnetic nanoantenna, results in the excitation of two mutually orthogonal localized plasmon oscillations lying in the plane perpendicular to the radiation wavevector, when a magnetic field is applied parallel to the propagation direction of the incident radiation. Besides its potential application to flat-optics nanodevices, the MO-induced optical activity of magnetic antennas has been employed to design ultrasensitive refractive index and molecular detectors, as well as magnetically-controllable rulers with an extreme sensitivity to the nanoscale distances.22,23 On the other hand, pure ferromagnetic nanoantennas with appreciable MOA are made of transition metals (e.g., nickel and cobalt), which display larger dielectric losses compared to conventional metals used in plasmonics.24-26 The intrinsic plasmonic damping of the constituent material results in broader and less intense LSPRs. To overcome the problem, various strategies have been applied so far, such as the fabrication of complex hybrid noblemetal/ferromagnetic structures or inducing coupling of broad localized modes with narrow excitations such as surface plasmon polaritons.27-33 Likewise, plasmonic crystals (PCs) based on ordered arrays of metallic antennas were shown to produce an exceptional sharpening of plasmonic resonant features. The presence of a periodic lattice structure gives rise to a collective resonant diffractive coupling through the far-fields scattered by the nanoantennas at particular wavelengths dictated by the geometry and periodicity of the PC, leading to the excitation of Fano resonances, so called lattice surface modes (LSMs).34-44 MO properties of periodic arrangements
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of MO-active antennas are starting to attract considerable attention. In particular, periodically arranged circular ferromagnetic nanoantennas have been studied very recently with the aim to exploit Fano LSMs to overcome the aforementioned limitation of the high losses and obtain narrow and intense MO resonant response.45, 46 Whilst the effects of lattice geometries and nanoantennas size and shape on the excitation of LSMs in noble-metal based PCs has been studied extensively34-44, their effects on the MO properties of PCs made of pure ferromagnetic elements are still unexplored. Here we generalize and broaden the implementation of PC concept to the magnetoplasmonics framework to design a magneto-plasmonic crystal (MPC) based on the excitation of anisotropic LSMs. We first show how the resonant excitation of in-plane LSMs can be conveniently and finely controlled by the anisotropic in-plane plasmonic response of the individual building blocks of the MPC, namely ferromagnetic elliptical nanodisks. Then, we demonstrate that the magnetic field activation of a second LSPR in the single disk, spectrally separated and orthogonal to that directly excited by the incident light, results in the simultaneous excitation of two perpendicular LSMs, one optically induced and the other MO-induced, which leads to an enhanced and highly tunable MOA. In more detail, we demonstrate that the enhancement and tunability of the MOA of the MPC can be strongly amplified and precisely molded around selected wavelengths through the lineshape tuning of the LSMs, achieved via the proper design of the shape and size of the individual antennas and of the periodicity of the lattice. This study sheds light on how to advantageously steer the synergistic combination of plasmonic anisotropy of the individual magnetic nanoantennas building the MPC and their diffractive coupling leading to a far richer physics and exploitable phenomenology of magneto-optical effects in nanoscaled magnetoplasmonic materials.
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We first analyze the key mechanism that governs the dependence of LSMs on the size of the individual building blocks of the MPC, whilst the period of the lattice, assumed to be square, is fixed. In the schematic shown in the top-left panel of Figure 1a the electric field Einc of normally-incident radiation is assumed to oscillate either along the principal x- or y-direction of a MPC made of plasmonic nanoantennas with in-plane aspect ratio (AR) equal to 1, that is circular disk shaped antennas. We consider that the nanoantennas have an edge-to-edge distance large enough (> 100 nm) that we can neglect near-field interactions in the visible and nearinfrared spectral region (see Supporting Information for detailed discussion and Figure S1). At these frequencies the electric field of the incident radiation induces dipolar LSPRs in each nanoantenna along its oscillation direction. LSMs arise from the onset of far-field in-plane diffractive coupling between the fields scattered by the individual antennas, according to the criterion developed by Rayleigh.47 In the spectral region of interest (550-1100 nm), the strongest diffractive interaction occurs in the in-plane direction perpendicular to that of the incident electric field, at a wavelength λR = a⋅n ( diffraction modes), where a is the array pitch in such a perpendicular direction and n is the refractive index of the surrounding medium. The wavelength λR corresponds to the photon energy at which the radiation scattered by the individual antenna is commensurate with the periodicity of the array and is related to the appearance of the first order diffracted radiation in the free space.36, 38, 46 Depending on the relative position between the LSPR of the single disk and λR, the diffractive coupling between the radiation fields produces Fano LSMs with different lineshapes.38 This is clearly shown in the calculated Extinction Efficiency Qext spectra plotted in the right-panel of Figure 1a, where we varied the diameter D of circular Ni disks, arranged in a periodic fashion (a = 500 nm along both x-and y-direction), from 100 to 200 nm. The thickness of the disks is 30 nm. The plotted curves
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were calculated using the Coupled Dipole Approximation (CDA) method (see Supporting Information for a detailed derivation), which is known to give an excellent description of the optical response of this kind of systems and it is based on the assumption that the individual antennas behave as a point-dipole.36 We observe a different lineshape depending on the position of the LSPR of the isolated disk (red curve in each frame ) with respect to λR (solid green line in each frame). It is worth noting that, despite the broad linewidth of the LSPRs of ferromagnetic nanoantennas, we can induce sharp LSMs even if their peaks are spectrally far from the diffraction edge: this enables a large degree of freedom in tuning the optical and MO properties of the MPC without changing the lattice parameter. Given the symmetry of the MPC considered here, it is clear that the LSM-modulated Qext does not change when the array is illuminated by either x- or y-polarized radiation impinging perpendicularly on the array plane, namely with the electric field oscillating along either x- or y-direction, respectively. If now the circular nanoantennas are replaced by elliptical shaped ones, the 90° rotational symmetry of the MPC is broken resulting in two different, spectrally separated LSPRs for x- and y-polarized incident radiation (or, equivalently, by rotating the sample by 90-degrees while retaining the light polarization). As a result two LSMs with different Fano lineshapes are excited depending on the relative orientation of the elliptical nickel particles with respect to the polarization of the incident light, as sketched in the left panel of Figure 1b. Additionally, in the case of ferromagnetic nanoantennas, the intrinsic MOA activated by the application of a magnetic field perpendicular to the crystal plane leads to the simultaneous and coherent excitation of two in-plane orthogonal LSPRs, as demonstrated for randomly arranged ferromagnetic elliptical nanodisks.20 The resonant diffractive coupling of these two antenna modes induced by the periodicity of the square lattice considered here, will then lead to the simultaneous excitation of two perpendicular
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LSMs with different Fano resonant lineshapes that can be precisely tailored through the design of the individual building blocks of the MPC. In order to verify the physical picture presented above, we first designed and fabricated using e-beam lithography (EBL) a sample made of randomly distributed elliptical nickel antennas on a glass substrate. The elliptical nanoantennas have an in-plane aspect ratio AR ≈ 2.5, to display LSPRs at markedly different wavelengths when the two axes are selectively excited by linearly polarized radiation impinging perpendicular to the sample surface. The in-plane dimensions of the antennas are 100 nm for the short axis (SA) and 250 nm for the long axis (LA). The thickness of the antennas is 30 nm. The system was then embedded in a homogenous medium with n = 1.5 after immersion in refractive index matching oil. A SEM image of a portion of the fabricated system is shown in the top-panel of Figure 2a. The Qext spectra of this sample for the two polarizations of the incident radiation are shown in the central-panel of Figure 2b (dashed blue line and dashed red line for light polarization along SA- and LA-direcrion, respectively). The spectra show broad peaks at 580 nm (polarization along SA) and 1050 nm (polarization along LA) arising from the excitation of the corresponding LSPRs. We then fabricated a MPC in which elliptical nanoantennas are arranged on a square array with a lattice constant a = 400 nm. We assume the LA and SA lying along the x- and ydirection, respectively, as sketched in the left-panel of Figure 1b. As for the random sample, the MPC was fabricated on a glass substrate and embedded in refractive index matching oil with n = 1.5. A SEM image of a portion of the fabricated system is shown in the bottom-panel of Figure 2a. The experimental Qext spectra of the MPC are plotted in the central-panel of Figure 2b with solid lines using the same color code as for the random sample. When the electric field of the incident radiation oscillates along the LA-direction (red curve), we observe the excitation of a
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LSM with a Fano lineshape featuring a dip around λR = a⋅n = 600 nm, followed by a broad peak in the near-infrared region close to 1000 nm. In contrast, when the incident electric field is parallel to the SA-direction (blue curve), a LSM is excited with a dip around the same λR but now followed by a narrow and sharp peak in the visible spectral region close to 650 nm. This behavior was expected based on the discussion of Figure 1 (see right-panel of Figure 1b). Moreover, it is well reproduced and interpreted theoretically by means of the CDA, which allows for a compact analytical formulation of the Qext spectra of periodic arrays of nickel nanoantennas assuming that they radiate as coupled oscillating dipoles.35,37 The Qext for the two polarizations of the incident field can be expressed as follows
∝ = ⁄
#
(1)
'
(2)
" !
∝ = $ ⁄
̅ " %% %% !
where and ( ) is the polarizability tensor elements of individual dipole along the LA (SA) ,, ,, / direction, (̃* is the permittivity of the embedding medium, and + ." (+ .") is the dipolar
far-field interaction factor for LA-(SA-) illumination (see Supporting Information). The last term depends only on the geometrical parameters of the lattice, and specifically on the y- (or x-) direction, indicated by the -. (or /.) unit vector dependence. From the experiments and the above formulas it is straightforward to see that the Qext spectrum of the MPC when the incident field is
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oscillating along the LA-direction, is determined by a LSM involving the antennas LA-LSPR and the far-field diffractive interaction along the SA-direction (as expressed by the y-direction ,, dependence of + ." and demonstrated in Supporting Information). Likewise, the Qext spectrum
for light polarized along the SA-direction is determined by a LSM resulting from the SA-LSPR excited in each antenna and the far-field diffractive interaction along the LA-direction. In the ,, / ,, present case of square lattices + ." = + .", so λR does not change and the lineshape of the
LSMs is determined by the relative position between the LSPR peak and the diffraction edge. The fact that the LSMs are determined by the far-field term of the dipole-dipole interaction along the direction perpendicular to the induced electric dipole in each nanoantenna, has indeed been observed experimentally in MPCs with cylindrical nanoantennas on a rectangular lattice.45 Two important observations can be made at this point regarding the optical properties of a MPC made of ferromagnetic nanoantennas: i) the periodic arrangement induces a marked sharpening of the spectral features near λR, as compared to the Qext of randomly distributed antennas48 and, more importantly, ii) the optical anisotropy ∆ induced in the system, defined as Qext-LA - Qext-SA (solid violet curve in the right-panel of Figure 2b), shows a clear enhancement close to the diffraction edge if compared to the randomly distributed nanoantennas case (dashed violet curve). The second effect, which can be seen in previously reported studies on PCs of anisotropic gold nanoantennas49, shows how the effects of the shape anisotropy of the individual building block on the optical properties of the system can be amplified and tuned via LSMs design. We then focused our attention on the MOA of our MPC, disclosing the most interesting effects arising from the application of an external magnetic field, as sketched in the left-panel of Figure 2c. When the incident electric field oscillates along the SA-direction and a magnetic field is applied perpendicularly to the crystal plane, the magnetic field activated SOC inherent to each
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individual nanoantenna induces a second oscillating dipole in the LA-direction with a resonant behaviour according to the LA-LSPR of the nanoantennas. The MOA of the MPC, namely the magnetic field induced change in the polarization of the reflected (or transmitted) light, is determined both by the electric field scattered by SOC-induced dipole along the LA-direction and by that originating from the dipole induced directly by the electric field of the incident radiation along the SA-direction. Due to phase lag between these two oscillating electric dipoles, the one directly excited by the incident electric field and the second one induced by the SOC20, the most convenient way to visualize the overall MO response is through the complex Kerr angle 2 , defined for each incidence direction (LA or SA) as: Φ
2 45 3 2 65 3
2 = 7 + i( =Φ
(3)
2 65 3 2 45 3
2 = 7 + i( =Φ
(4)
where the real (θ) and imaginary (ε) parts are the MO-induced rotation (with respect to the incident polarization plane) and ellipticity of the outward reflected (or transmitted) electric field, respectively. Physically, the modulus of the complex Kerr angle is representative of the total MOA of the system and it is defined as33
2 | = 97 : +( : |Φ
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Following the same reasoning used for explaining the Qext spectra in Figure 2b, we now expect the MOA to be determined by MO-LSMs stemming from diffractive coupling of the electric fields emitted by the SOC-induced dipoles and acting in the direction perpendicular to them, namely parallel to the incident radiation polarization, as sketched in the left-panel of Figure 2c. The experimental MOA spectra measured from our MPC, after saturating the magnetization of the nanoantennas by applying a magnetic field |Hsat| = 3 kOe perpendicular to the MPC plane, are shown in the central-panel of Figure 2c, where the red and blue curves refer to an incident electric field oscillating along the LA- and SA-direction, respectively. It is worth noting how the MOA features notably resemble those of the Qext measured with the polarization of the incident field along the perpendicular direction. Similar, but configurationally different effects between optical and magneto-optical spectra have also been observed in MPCs made of rectangular arrays of circular Ni nanodisks.45 The experimental results in the central-panel of Figure 2c confirm the intuitive picture shown in the cartoon in the left-panel and are well reproduced by the following analytical formulas governing the MOA complex angles (see Supporting Information for the complete derivation): 2 ≈
